This invention relates to an image display apparatus which includes a pixel whose brightness is controlled with a signal, and more particularly to an image display apparatus which includes, for each pixel, a light emitting element for emitting light with brightness which is controlled with current such as an organic electroluminescence (EL) element. More specifically, the present invention relates to an image display apparatus of the active matrix type wherein the amount of current to be supplied to a light emitting element is controlled by an active element such as a field effect transistor of the insulated gate type provided in each pixel.
Generally, in an image display apparatus of the active matrix type, a large number of pixels are arranged in a matrix, and the intensity of light is controlled for each of the pixels in response to brightness information given thereto to display an image. Where liquid crystal is used as an electro-optical substance, the transmission factor of each pixel varies in response to a voltage written in the pixel. Even with an image display apparatus of the active matrix type which employs an organic electroluminescence material as an electro-optical substance, basic operation is similar to that where liquid crystal is employed. However, different from a liquid crystal display apparatus, an organic EL display apparatus is an apparatus of the self light emission type wherein each pixel has a light emitting element. Thus, the organic EL display apparatus is advantageous in that it exhibits a higher degree of visibility than a liquid crystal display apparatus, that it does not require a back light and that it has a higher responding speed. The brightness of each individual light emitting element is controlled with the amount of current. In other words, the organic EL display is significantly different from the liquid crystal display apparatus and so forth in that the light emitting elements are of the current driven type or the current controlled type.
Similarly to the liquid crystal display apparatus, the organic EL display apparatus can possibly use a simple matrix system or an active matrix system as a driving system therefor. Although the former is simple in structure, it is difficult to implement a display apparatus of a large size and a high resolution. Therefore, much effort has been and is directed to development of organic EL display apparatus of the active matrix system. In the organic EL display apparatus of the active matrix system, current to flow to a light emitting element provided in each pixel is controlled by an active element usually in the form of a thin film transistor which is a kind of a field effect transistor of the insulated gate type and may be hereinafter referred to as TFT. An organic EL display apparatus of the active matrix system is disclosed, for example, in Japanese Patent Laid-open No. Hei 8-234683, and an equivalent circuit for one pixel in the organic EL display apparatus is shown in FIG. 10. Referring to FIG. 10, the pixel PXL shown includes a light emitting element OLED, a first thin film transistor TFT1, a second thin film transistor TFT2, and a holding capacitor Cs. The light emitting element OLED is an organic electroluminescence (EL) element. Since an organic EL element in most cases has a rectification property, it is often called OLED (organic light emitting diode) and, in FIG. 10, the mark of a diode is used for the light emitting element OLED. However, the light emitting element is not limited to an OLED, but may be any element only if the brightness thereof is controlled with the amount of current to flow therethrough. It is not always required for an OLED to have a rectification property. In the pixel shown in FIG. 10, a reference potential (ground potential) is applied to the source S of the second thin film transistor TFT2, and the anode A (positive electrode) of the light emitting element OLED is connected to a power supply potential Vdd while the cathode K (negative electrode) is connected to the drain D of the second thin film transistor TFT2. Meanwhile, the gate G of the first thin film transistor TFT1 is connected to a scanning line X and the source S of the first thin film transistor TFT1 is connected to a data line Y. The drain D of the first thin film transistor TFT1 is connected to the holding capacitor Cs and the gate G of the second thin film transistor TFT2.
In order to cause the pixel PXL to operate, the scanning line X is placed into a selected state first, and then a data potential Vdata representative of brightness information is applied to the data line Y. Consequently, the first thin film transistor TFT1 is rendered conducting, and the holding capacitor Cs is charged or discharged and the gate potential of the second thin film transistor TFT2 becomes equal to the data potential Vdata. Then, if the scanning line X is placed into a non-selected state, then the first thin film transistor TFT1 is turned off, and the second thin film transistor TFT2 is electrically disconnected from the data line Y. However, the gate potential of the second thin film transistor TFT2 is held stably by the holding capacitor Cs. The current flowing to the light emitting element OLED through the second thin film transistor TFT2 exhibits a value which depends upon a gate-source voltage Vgs of the second thin film transistor TFT2, and the light emitting element OLED continues to emit light with a brightness value corresponding to the amount of current supplied from the second thin film transistor TFT2.
In the present specification, the operation of selecting a scanning line X to transmit a potential of a data line Y to the inside of a pixel is hereinafter referred to as xe2x80x9cwritexe2x80x9d. Where the current flowing between the drain and the source of the second thin film transistor TFT2 is represented by Ids, this is driving current flowing to the light emitting element OLED. If it is assumed that the second thin film transistor TFT2 operates in a saturation region, then the current Ids is represented by the following expression:                                                         Ids              =                                                (                                      1                    /                    2                                    )                                ·                μ                ·                Cox                ·                                  (                                      W                    /                    L                                    )                                ·                                                      (                                          Vgs                      -                      Vth                                        )                                    2                                                                                                        =                                                (                                      1                    /                    2                                    )                                ·                μ                ·                Cox                ·                                  (                                      W                    /                    L                                    )                                ·                                                      (                                          Vdata                      -                      Vth                                        )                                    2                                                                                        (        1        )            
where Cox is a gate capacitance per unit area and is given by the following expression:
xe2x80x83Cox=xcex50xc2x7xcex5r/dxe2x80x83xe2x80x83(2)
In the expressions (1) and (2) above, Vth is a threshold voltage for the second thin film transistor TFT2, xcexc is the mobility of carriers, W is the channel width, L is the channel length, xcex5 0 is the dielectric constant of vacuum, xcex5 r is the dielectric constant of the gate insulating film, and d is the thickness of the gate insulating film.
According to the expression (1), the current Ids can be controlled with the data potential Vdata to be written into the pixel PXL, and as a result, the brightness of the light emitting element OLED can be controlled. Here, the reason why the second thin film transistor TFT2 operates in a saturation region is such as follows. In particular, the reason is that, since, in a saturation region, the current Ids is controlled only with the gate-source voltage Vgs but does not rely upon the drain-source voltage Vds, even if the drain-source voltage Vds is fluctuated by a dispersion in characteristic of the light emitting element OLED, a predetermined amount of current Ids can be flowed to the light emitting element OLED.
As described hereinabove, with the circuit construction of the pixel PXL shown in FIG. 10, if writing of the data potential Vdata is performed once, then the light emitting element OLED continues to emit light with a fixed brightness value for a period of one scanning cycle (one frame) until it is rewritten. If a large number of such pixels PXL are arranged in a matrix as shown in FIG. 11, then an image display apparatus of the active matrix type can be constructed. As seen from FIG. 11, a conventional image display apparatus includes a plurality of scanning lines X1 to XN for selecting pixels PXL in a predetermined scanning cycle (for example, in a frame period complying with the NTSC standards), and a plurality of data lines Y for providing brightness information (data potentials Vdata) for driving the pixels PXL. The scanning lines X1 to XN and the data lines Y extend perpendicularly to each other such that the pixels PXL may be arranged in a matrix at intersecting points thereof. The scanning lines X1 to XN are connected to a scanning line drive circuit 21, and the data lines Y are connected to a data line drive circuit 22. The scanning lines X1 to XN are successively selected by the scanning line drive circuit 21 while writing of the data potentials Vdata is repeated successively from the data lines Y by the data line drive circuit 22 thereby to display a desired image. While, in an image display apparatus of the simple matrix type, the light emitting element included in each pixel PXL emits light only at a selected instant, the image display apparatus of the active matrix type shown in FIG. 11 is advantageous in that, since the light emitting element of each pixel PXL continues its light emission also after writing into it is completed, the peak brightness (peak current) of the light emitting elements can be decreased when compared with that of the image display apparatus of the simple matrix type, particularly where the display device has a large size and a high resolution.
FIG. 12 is an equivalent circuit diagram showing another conventional pixel structure. In FIG. 12, elements corresponding to those of the conventional pixel structure shown in FIG. 10 are denoted by like reference characters to facilitate understanding. While the conventional pixel structure of FIG. 10 uses a field effect transistor of the N-channel type for the thin film transistors TFT1 and TFT2, the conventional pixel structure of FIG. 12 uses a field effect transistor of the P-channel type. Accordingly, in the pixel structure of FIG. 12, the cathode K of the light emitting element OLED is connected to the negative power supply potential Vdd and the anode A is connected to the drain D of the second thin film transistor TFT2 conversely to those in the pixel structure of FIG. 10.
FIG. 13 is a cross sectional view schematically showing a sectional structure of the pixel PXL shown in FIG. 12. However, in order to facilitate illustration, only the light emitting element OLED and the second thin film transistor TFT2 are shown in FIG. 13. The light emitting element OLED includes a transparent electrode 10, an organic EL layer 11 and a metal electrode 12 placed one on another in this order. The transparent electrode 10 is provided separately for each pixel and functions as the anode A of the light emitting element OLED, and is formed from a transparent conductive film of, for example, ITO. The metal electrode 12 is connected commonly among the pixels and functions as the cathode K of the light emitting element OLED. In particular, the metal electrodes 12 are connected commonly to a predetermined power supply potential Vdd. The organic EL layer 11 is a composite film including, for example, a positive hole transporting layer and an electron transporting layer. For example, Diamyne is vapor deposited as the positive hole transporting layer on the transparent electrode 10 which functions as the anode A (positive hole injecting electrode) and Alq3 is vapor deposited as the electron transporting layer on the positive hole transporting layer, and then the metal electrode 12 which functions as the cathode K (electron injecting electrode) is formed on the electron transporting layer. It is to be noted that Alq3 represents 8-hydroxy quinoline aluminum. The light emitting element OLED having such a layered structure as just described is a mere example at all. If a forward voltage (approximately 10 V) is applied between the anode and the cathode of the light emitting element OLED having such a structure as described above, then injection of carriers such as electrons and positive holes occurs, and emission of light is observed. The operation of the light emitting element OLED is considered to be emission of light by excited elements formed from positive holes injected from the positive hole transporting layer and electrons injected from the electron transporting layer.
Meanwhile, the second thin film transistor TFT2 includes a gate electrode 2 formed on a substrate 1 made of glass or the like, a gate insulating film 3 placed on the upper face of the gate electrode 2, and a semiconductor thin film 4 placed on the gate electrode 2 with the gate insulating film 3 interposed therebetween. The semiconductor thin film 4 is formed from, for example, a polycrystalline silicon thin film. The second thin film transistor TFT2 includes a source S, a channel Ch and a drain D which form a path for current to be supplied to the light emitting element OLED. The channel Ch is positioned immediately above the gate electrode 2, and the second thin film transistor TFT2 of the bottom gate structure is covered with an interlayer insulating film 5, and a source electrode 6 and a drain electrode 7 are formed on the interlayer insulating film 5. The light emitting element OLED described above is formed on the elements mentioned above with another interlayer insulating film 9 interposed therebetween.
The first subject to be solved when such an EL display apparatus of the active matrix type as described above is to be formed is that the degree of freedom in designing the second thin film transistor TFT2 which is an active element for controlling the amount of current to flow through the light emitting element OLED is low and, under certain circumstances, practical designing suitable for pixel dimensions is difficult. The second subject to be solved is that it is difficult to freely adjust the display brightness of the entire screen. The subjects described are described giving specific design parameters with regard to the conventional apparatus described above with reference to FIGS. 10 to 13. In a typical design example, the screen size is 20 cmxc3x9720 cm, the number of rows (scanning line number) 1,000, the number of columns (data line number) 1,000, the pixel size S=200 xcexcmxc3x97200 xcexcm, the peak brightness Bp=200 cd/m2, the efficiency of the light emitting element E=10 cd/A, the thickness of the gate insulating film of the second thin film transistor TFT2 d=100 nm, the dielectric constant of the gate insulating film xcex5 r=3.9, the carrier mobility xcexc=100 cm2/Vxc2x7S, the peak current per pixel Ip=Bp/Exc3x97S=0.8 xcexcA, the peak value of |Vgsxe2x88x92Vth| (driving voltage) Vp=5 V. In order to supply the peak current Ip in the design example above, as a design example of the second thin film transistor TFT2, the channel width and the channel length are determined from the expressions (1) and (2) given hereinabove as follows:                                                         Channel  width:                        ⁢                          xe2x80x83                        ⁢            W                    =                      5            ⁢                          xe2x80x83                        ⁢            µm                          ⁢                  
                ⁢                              Channel  length:                    ⁢                      xe2x80x83                    ⁢                                                                      L                  =                                      xe2x80x83                                    ⁢                                                            {                                              W                        /                                                  (                                                      2                            ·                            Ip                                                    )                                                                    }                                        ·                    μ                    ·                    Cox                    ·                                          Vp                      2                                                                                                                                            =                                      xe2x80x83                                    ⁢                                      270                    ⁢                                          xe2x80x83                                        ⁢                    µm                                                                                                          (        3        )            
Here, it is the first problem that the channel length L given by the expression (3) above is equal to or greater than the pixel size (S=200 xcexcmxc3x97200 xcexcm). As seen from the expression (3), the peak current Ip increases in inverse proportion to the channel length L. In the example described above, in order to suppress the peak current Ip to approximately 0.8 xcexcA which is sufficient for operation, the channel length L must be set long to 270 xcexcm. However, this is not preferable because it requires a large occupied area of the TFT2 in the pixel, resulting in reduction of the light emitting area. Besides, refinement of pixels becomes difficult. The essential problem resides in that, if a brightness value (peak current) required and parameters of a semiconductor process and so forth are given, then there is little degree of freedom in designing of the second thin film transistor TFT2. In particular, a possible idea for reducing the channel length L in the example described above is to reduce the channel width w as can be seen apparently from the expression (3). However, there is a limitation to refinement of the channel width W in terms of the process, and it is difficult to refine the channel width W significantly with respect to the degree described above in a thin film transistor process at present. It is another possible idea to reduce the peak value Vp of the driving voltage. In this instance, however, in order to perform gradation control, it is necessary to control the intensity of light to be emitted from the light emitting element OLED with a very small driving voltage step. For example, also in the case of the peak value Vp=5 V, if it is tried to control the intensity of light to be emitted with 64 gradations, then the voltage step per one gradation is approximately 5 V/64=80 mV in average. If the voltage step is further reduced, then the display quality of the image display is influenced by fine noise or a dispersion of the TFT character. Accordingly, there is a limitation also to reduction of the peak value Vp of the driving voltage. Another possible solution is to set process parameters such as the carrier mobility xcexc appearing in the expression (3) to suitable values. However, it is generally difficult to control process parameters to preferable values with a high degree of accuracy, and economically, it is quite unrealistic to construct a production process in accordance with specifications of an image display apparatus to be designed at all. In this manner, in a conventional EL display apparatus of the active matrix type, the degree of freedom in designing of a pixel is so low that it is difficult to perform practical designing.
In relation to the first problem described above, it is a second problem that, in an EL display apparatus of the active matrix type, it is difficult to arbitrarily control the display brightness of the entire screen. Generally, in an image display apparatus of a television set or the like, it is an essential requirement for practical use that the display brightness of the entire screen can be adjusted freely. For example, it is natural to set the screen brightness high when the image display apparatus is used in a light situation, but suppress the screen brightness low conversely when the image display apparatus is used in a dark situation. Such adjustment of the screen brightness can be realized readily by, for example, with a liquid crystal display, varying the power of the backlight. On the other hand, with an EL display apparatus of the simple matrix type, the screen brightness can be adjusted comparatively simply by adjusting the driving current upon addressing.
However, with an organic display apparatus of the active matrix type, it is difficult to arbitrarily adjust the display brightness of the entire screen. As described above, the display brightness increases in proportion to the peak current Ip, and the peak current Ip increases in inverse proportion to the channel length L of the TFT2. Accordingly, in order to lower the display brightness, the channel length L should be increased. This, however, cannot be employed as a countermeasure for selecting the display brightness arbitrarily by a user. A method which seems possible to realize is to reduce the peak value Vp of the driving voltage in order to reduce the brightness. However, if the peak value Vp is reduced, then deterioration of the picture quality is caused by noise or the like. On the contrary where it is desired to raise the brightness, even if it is tried to raise the peak value Vp of the driving voltage, it is a matter of course that there is an upper limitation to it because of a voltage withstanding property of the second thin film transistor TFT2 and so forth.
It is an object of the present invention to provide an image display apparatus which increases the degree of freedom in designing of an active element in the inside of a pixel to allow good designing and can adjust the screen brightness freely and simply.
In order to attain the object described above, according to a first aspect of the present invention, there is provided an image display apparatus, comprising a plurality of pixels arranged in a matrix, a plurality of scanning lines for selecting the pixels in a predetermined scanning cycle, a plurality of data lines extending perpendicularly to the scanning lines for providing brightness information to drive the pixels, the pixels being disposed at intersecting points of the scanning lines and the data lines, each of the pixels including a light emitting element for emitting light with a brightness value which varies depending upon an amount of current supplied thereto, a first active element controlled by one of the scanning lines for writing the brightness information given thereto from one of the data lines into the pixel, and a second active element for controlling the amount of current to be supplied to the light emitting element in response to the brightness information written in the pixel, writing of the brightness information into each of the pixels being performed by applying an electric signal corresponding to the brightness information to the data line connected to the pixel while the scanning line connected to the pixel is selected, the brightness information written in each of the pixels being held by the pixel also after the scanning line connected to the pixel is placed into a non-selected state so that the light emitting element of the pixel can continue lighting with a brightness value corresponding to the brightness information held by the pixel, and control means for compulsorily extinguishing the light emitting elements of those of the pixels which are connected to a same one of the scanning lines at least in a unit of a scanning line so that the light emitting elements are placed into an extinguished state from a lit state within a period of one scanning cycle after the brightness information is written into the pixels until new brightness information is written into the pixels subsequently.
Preferably, the control means is capable of adjusting a point of time at which each of the light emitting elements is changed over from a lit state to an extinguished state within a period of one scanning cycle after the brightness information is written into the pixels until new brightness information is written into the pixels subsequently.
The image display apparatus may be constructed such that the control means includes a third active element connected to a gate of the second active element, which is in the form of a field effect transistor of the insulated gate type, of each of the pixels and is capable of providing a control signal to the third active element to control a gate potential of the second active element thereby to extinguish the light emitting element of the pixel, the control signal being applied to the third active elements included in those of the pixels which are on a same one of the scanning lines over a stopping control line provided for and in parallel to each of the scanning lines.
As an alternative, the image display apparatus may be constructed such that the control means includes a third active element connected in series to the light emitting element of each of the pixels and is capable of providing a control signal to the third active element to cut off current to flow to the light emitting element, the control signal being applied to the third active elements included in those of the pixels which are on a same one of the scanning lines over a stopping control line provided for and in parallel to each of the scanning lines.
Otherwise, the image display apparatus may be constructed such that the light emitting element of each of the pixels includes a two-terminal element having a rectification function and having a first terminal connected to the second active element and a second terminal connected to the second terminals of those of the pixels which are connected to a same one of the scanning lines to which the pixel is connected but electrically isolated from the second terminals of those of the pixels which are connected to any other one of the scanning lines, and the control means controls a potential of the second terminals of the two-terminal elements which are connected commonly to the same scanning line to extinguish the two-terminal elements.
The control means may select, within a period of one scanning cycle after the brightness information is written into the pixels until new brightness information is written into the pixels subsequently, the scanning lines again to write information representative of brightness of zero into the pixels from the data lines to extinguish the light emitting elements of the pixels.
The image display apparatus may be constructed otherwise such that each of the pixels further includes a capacitive element having an end connected to a gate of a field effect transistor of the insulated gate type which forms the second active element for controlling the amount of current to flow to the light emitting element, and the control means controls a potential of the other end of the capacitive element to control a potential of the gate of the field effect transistor of the insulated gate type which forms the second active element to extinguish the light emitting element.
The control means may otherwise control a lighting point of time and an extinguishing point of time of the light emitting element included in each of the pixels at least in a unit of a scanning line within one scanning cycle after the brightness information is written into the pixel.
The image display apparatus may be constructed otherwise such that pixels for red, green and blue are connected commonly to each of the scanning lines, and the control means extinguishes the light emitting elements included in the pixels for red, green and blue at different points of time from one another.
Preferably, the light emitting element is an organic electroluminescence element.
According to a second aspect of the present invention, there is provided an image display apparatus wherein a plurality of pixels are lit in response to brightness information within a period of one scanning cycle after first brightness information is written into the pixels until new second brightness information is written into the pixels, comprising a plurality of scanning lines for individually selecting the pixels in a predetermined scanning cycle, a plurality of data lines formed perpendicularly to the scanning lines for providing brightness information for lighting the pixels, a first active element controlled by each of the scanning lines for fetching the brightness information into each of the pixels, a second active element for converting the brightness information fetched by the first active element into an electric signal to be used to drive the pixel, and control means for placing the pixels from a lit state into an extinguished state within the period of one scanning cycle.
Preferably, the control means is capable of varying a time after the pixels are lit until the pixels are extinguished within the period of one scanning cycle.
The image display apparatus may be constructed such that the second active element is a field effect transistor of the insulated gate type, and the control means includes a third active element connected to a gate of the field effect transistor of the insulated gate type and controlled over a control line which is provided substantially in parallel to each of the scanning lines.
The control means may include a third active element provided in series to the second active element and controlled over a control line which is provided substantially in parallel to each of the scanning lines.
The image display apparatus may be constructed otherwise such that each of the pixels includes a light emitting element having a first terminal connected to the second active element and a second terminal connected to a reference potential, and the control means variably controls the reference potential to extinguish the light emitting element.
The control means may select, after the scanning lines are selected, the scanning lines again within the period of one scanning cycle and supply the brightness information which represents brightness of zero from the data lines to the pixels to extinguish the pixels.
The image display apparatus may be constructed otherwise such that each of the pixels includes a capacitive element having an end connected to a gate of a field effect transistor of the insulated gate type which forms the second active element, and the control means controls a potential of the other end of the capacitive element to control a potential of the gate of the field effect transistor of the insulated gate type which forms the second active element to extinguish the pixels.
The control means may extinguish the pixels for each of the scanning lines.
The image display apparatus may be constructed otherwise such that each of the pixels includes light emitting elements for blue, green and red, and the control means is capable of extinguishing the light emitting elements for blue, green and red at different times from one another.
The image display apparatus may be constructed further otherwise such that the second active element converts the brightness information into current to be used for driving of the pixels, and each of the pixels includes a light emitting element which makes use of an organic substance which emits light with current.
The image display apparatus may be constructed otherwise such that it further comprises a scanning line drive circuit to which a vertical clock signal for successively selecting the scanning lines is inputted, and that the control means includes a control circuit for receiving another vertical clock signal obtained by delaying the vertical clock signal by a predetermined period to select the scanning lines or control lines provided in parallel to the scanning lines, and the scanning lines are successively selected in synchronism with the vertical clock signal by the scanning line drive circuit to light the pixels, the pixels which have been lit being extinguished over the scanning line or the control lines within the period of one scanning cycle in synchronism with the delayed vertical clock signal by the control circuit. In this instance, the image display apparatus may be constructed further such that it further comprises a data line drive circuit for providing the brightness information to the data lines, and that each of outputs of the scanning line drive circuit is connected to an input terminal of a logical OR circuit having an output terminal connected to one of the scanning lines while each of outputs of the control circuits is connected an input terminal of a logical AND circuit connected to the other input terminal of the logical OR circuit, and the vertical clock signal is inputted to the other input terminal of the logical AND circuit.
In the image display apparatus, after brightness information is written into the pixels in a unit of a scanning line, the light emitting elements included in the pixels are extinguished collectively in a unit of a scanning line before brightness information of a next scanning line cycle (frame) is newly written into the pixels. Or in other words, after brightness information is written into each pixel and the pixel begins to emit light, the emission of light can be stopped before writing of a next frame is performed. Consequently, the time from lighting to extinction of the light emitting elements after brightness information is written into the pixels can be adjusted. In other words, the ratio (duty) of the time of light emission within one scanning cycle or one frame can be adjusted. The adjustment of the time of light emission (duty) corresponds to adjustment of the peak current of each light emitting element. Therefore, by adjusting the duty, the display brightness, that is, the display brightness average in time, can be adjusted simply and freely. What is more significant is that the peak current can be increased by setting the duty appropriately. For example, if the duty is reduced to {fraction (1/10)}, then an equal brightness value is obtained even if the peak current is increased to 10 times. If the peak current is increased to 10 times, then the channel length of a thin film transistor included in each pixel can be reduced to {fraction (1/10)}. In this manner, by suitably selecting the duty, the degree of freedom in designing a thin film transistor included in each pixel increases, and this allows practical designing. Further, since the duty can be set freely, a degree of freedom is provided in that the amount of current to flow to each light emitting element upon light emission is set suitably while the display brightness average in time is kept equal. Consequently, a degree of freedom in designing of an active element for controlling the amount of current to flow to the light emitting element is produced. As a result, it becomes possible to design an image display apparatus which can provide an image of a higher degree of picture quality or another image display apparatus of a smaller pixel size.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.